Superconductivity is a phenomenon where electrical resistance of outer applied voltage with respect to current drops to zero under a certain temperature and a certain magnetic field so that electrons in a copper pair are formed according to BCS theory, and as a result, loss of heat caused by the resistance disappears. With many kinds of metals, electrical resistance suddenly drops to zero under a low temperature of about −270˜−196° C. At this time, this kind of material refers to a superconductor, and a temperature and a magnetic field which allow superconductivity to occur refer to a ‘critical temperature’ and a ‘critical magnetic field,’ respectively.
In general, all materials include spin magnets which are entirely attracted by a magnet while being arranged in a direction of an external magnetic field. Typical material, which has much less effect that such spin magnets are arranged in a magnetic field direction so that a phenomenon of the material being attracted by a magnet is rarely observed in a normal state, refers to a paramagnetic material, and material in which such a characteristic is superior so that the material is easily attracted by a magnet, i.e. material such as iron, refers to a ferromagnetic material.
Also, a material, whose internal electrons allows induced current caused by electromagnetic induction to flow under influence of an external magnetic field due to absence of spin magnets so as to intercept an external magnetic field so that the material is forced in such a direction that it is repelled from a magnet, refers to diamagnetic material.
In a case where such a superconductor is used as a coil, since loss of energy doesn't occur, an electromagnet, which can make a strong magnetic field under a small amount of current, can be made, and since a superconductor is a diamagnetic substance, if a magnet is located on the superconductor, a magnetic field caused by the magnet cannot pass through the superconductor, but is repelled from the superconductor so that the magnet can be levitated.
The main characteristic of a superconductor, which allows such superconductivity to occur, is that it is nonresistance-substance that does not have resistance disturbing current flow under a certain temperature and a certain magnetic field, but it is a diamagnetic substance which does not allow a magnetic field to pass through, and it can accept an external magnetic field so as to be in a state where a superconducting state is mixed with a normal state.
A superconductor is divided into a type I superconductor, which has an electric resistance of zero and also has a strong diamagnetic characteristic so as to completely cancel an external magnetic field so that an internal magnetic field of the superconductor drops to zero, and a type II superconductor, which accepts an external magnetic field based on a certain limited value, so that a superconducting state is broken and mixed with a normal state.
The type I superconductor includes most pure metals and its characteristic appears when an outer magnetic field (H) is smaller than a critical magnetic field (Hc). The type I superconductor allows super current to flow along a surface of the superconductor but doesn't allow current to flow into the interior of the superconductor at a depth deeper than a predetermined depth, so that an inner magnetic field disappears. The type I superconductor exhibits a meissner effect of superconductive current flow so that an inner magnetic field flows in an opposite direction of an external magnetic field so as to cancel the external magnetic field.
The type II superconductor includes Nb3Sn, Nb3Al, NbTi, MgB2, a high temperature superconductor, etc., and shows a strong magnetic field. The type II superconductor impels an outer magnetic field to a lower critical magnetic field Hc1 so that a diamagnetic state where a magnetic field doesn't exist at the interior of a superconductor is achieved, but accepts an outer magnetic field (H) little by little between a lower critical magnetic field (Hc1) and an upper critical magnetic field (Hc2) so that the superconductor starts breaking bit by bit and generating countless vortexes in a normal state.
Also, in the type II superconductor, a state where a superconductive property and a vortex are mixed with each other is formed so that two electrons in a copper pair are formed. Therefore, there are a superconductor allowing superconductive current to flow along a peripheral surface of the vortex and another superconductor allowing superconductive current to flow without electric resistance while forming two electrons in a copper pair by current applied from an external.
Particularly, the type II superconductor has a superior superconductive characteristic since it has a big flux pinning (vortex pinning) effect where a vortex is prevented from moving, but when its magnetic field is over an upper critical magnetic field (Hc2), the type II superconductor returns to a normal state while superconductivity is broken.
Also, a superconductor can be divided into a high temperature superconductor and a low temperature superconductor according to a temperature at which it is used. The former allows superconductivity at a temperature of liquid nitrogen (77K), and the latter allows superconductivity at a temperature of liquid helium (4K). More than one thousand kinds of such superconductors are discovered from a metal, organic matter, ceramic, chemical compound, etc. Nb—Ti alloy which is a metal-based superconductive material, and Nb3Sn, which is chemical compound-based superconductive material, have recently been put to practical use, and they are used in a tokamak device for a fusion reactor, a particle accelerator, a medical MRI, an analytic NMR, etc.
So as to make a magnet which can create a very large magnetic field by using superconductors which are used in various fields as described above, a superconducting wire, which has a superior critical current (IC) as well as a strong characteristic such as critical current density (JC) in a ferromagnetic field, is required. A typical superconducting wire can be a metallic compound Nb3Sn wire which has been manufactured by various methods, such as an internal diffusion method, a bronze method, etc.
According to the internal diffusion method, as shown in FIGS. 1 to 3, niobium filaments 12 are inserted into and are properly arranged at a proper interior position of one 11 of a copper rod and a copper alloy rod having copper as a matrix in an axial direction so as to form an extrusion billet 1, and then the extrusion billet 1 is extruded so as to form an extrusion rod.
Then, a hole is bored through a central part of the extrusion rod, one 13 of a tin rod and a tin alloy rod is inserted into the hole, and then a drawing process is repeatedly performed several times so as to manufacture a subelement 2. A plurality of modules 2 made by cutting the subelement 2 into a proper length and cleaning it are collectively arranged at the interior of a diffusion preventing tube 33 made from tantalum or niobium, etc., and then a spacer 32 is inserted into each space between the modules 2. Therefore, a restacking billet 3 is formed according to such a scheme.
At this time, the diffusion prevention tube 33 is assembled with an inner circumferential surface of the stabilizing tube 31 made from copper or copper alloy while making contact therewith.
The restacking billet made as described above undergoes heat-treatment after a drawing process performed several tens of times so that reciprocal diffusion reaction occurs between any one of a copper rod and a copper alloy rod and niobium filaments due to the heat treatment, thereby forming an Nb3Sn chemical compound, which is a superconductor.
At this time, the spacers 32 inserted into the interior of the restacking billet 3 are used to minimize space necessarily formed between the modules 2. In the conventional art, a tin-based spacer such as a tin rod, a tin alloy rod, etc. is used, and it is typical that a plurality of modules 2 having a circle-sectional shape is inserted into the diffusion prevention tube 33, and then the spacer 32 is inserted into only the largest space among spaces formed between the modules 2.
Therefore, when the restacking billet having spacers inserted therein undergoes a drawing process several tens of times, each space between the modules disappears while being compressed so that a sectional shape of each subelement is changed from a circular shape to a hexagonal shape. Through such a series of drawing processes, inner stress of each subelement becomes irregular, and as a result, wire cutting occurs during a drawing process so that a problem is generated in manufacturing a wire having a long length, and manufacturing costs also increase.
As an example of the above described method, a method for manufacturing an A15 type Nb3Sn superconducting wire by using an internal diffusion method is disclosed in Japan Patent NO. 4-129106. In this method, six tin alloy spacers are inserted into the outermost layer at which seven modules are stacked, and a tin alloy spacer having a small wire diameter is inserted into each space having a section of a roughly triangular shape, which is formed between three modules. While a corner part is formed at an outer periphery of each module, each niobium filament is changed into a flat shape, and not into a circular shape. In a magnet integrally formed through a stranded cable process for twisting several superconducting wires into several ones, a cabling, a welding process, etc., deformation-resistance against a repetition cycle in compression and extension generated under a specific environment causing superconductivity, particularly under an extremely low temperature and a high magnetic field, becomes weaker. Therefore, deterioration of a superconductive characteristic, such as reduction of a critical current of a superconducting wire, n-value, etc., is caused.
Also, when each shape of the niobium filaments is irregularly changed into a flat shape, and not into a circular shape, a critical current density is remarkably reduced due to an outer magnetic field, and magnetization loss respective to an applied magnetic field is generated. Therefore, there is a limitation in the application of a superconducting wire generating a strong alternating current magnetic field, and a superconductive characteristic respective to strain caused by a repetition cycle in extension and compression under an extremely low temperature is remarkably deteriorated.
Also, due to an excessive amount of a tin element, as a superconductive material generated through undergoing a drawing and a heating process, unusual superconductive material such as Nb6Sn5 or NbSn2, etc. and not Nb3Sn, is generated so that a critical current is too lowered.